Our studies are focused on the expression of growth hormone and prolactin genes in normal and transformed pituitary cells and are designed to show how the specific methylation of mRNA molecules affects the processing of nuclear mRNA precursors into mature cytoplasmic sequences. Our approach to these studies involves obtaining full length cDNA clones corresponding to bovine growth hormone and bovine prolactin mRNAs as well as genomic clones for both genes. The cDNA clones are used in hybridization studies to follow the cellular fate of mature mRNAs and in the characterization of mRNA methylation patterns, whereas the genomic clones provide a source of intron-specific probes to identify nuclear precursors to mRNA and are also used for the insertion of extra copies of these genes into normal and SV40-transformed cells. In very recent studies, we have shown that an inhibitor which blocks mRNA methylation dramatically impairs the cytoplasmic entry of undermethylated mRNA molecules. The availability of these cloned DNA sequences and this effective inhibitor of mRNA methylation now permits us to examine the function of this ubiquitous mRNA modification in the regulation of gene expression in normal and transformed cells.